microsoft
diff --git a/‎.github/scripts/merge_coverage.py‎
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+#!/usr/bin/env python3
+"""Merge per-platform llvm-cov JSON exports into a per-line set-union report.
+
+Usage:
+  merge_coverage.py --output-json OUT.json --output-md OUT.md \\
+      LABEL=PATH [LABEL=PATH ...]
+
+Each PATH is a coverage.json from `llvm-cov export -format=json`. LABEL is
+a short platform identifier (e.g. ``linux``, ``macos``, ``selfhost-shim``).
+
+Outputs:
+  OUT.json  per-file (executable, covered) line sets, plus per-platform map.
+  OUT.md    markdown summary suitable for a PR comment.
+
+The per-line set-union design and its invariant (covered(f) ⊆ executable(f))
+are documented inline.
+"""
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+from pathlib import Path
+
+
+# --- Schema constants (llvm-cov JSON segment format) -------------------------
+# Segment = [line, col, count, has_count, is_region_entry, is_gap_region]
+_SEG_LINE = 0
+_SEG_COUNT = 2
+_SEG_HAS_COUNT = 3
+_SEG_IS_GAP = 5
+_SEG_MIN_LEN = 6
+
+
+def normalise_path(path: str) -> str:
+    """Normalise a filename so the same source file from different CI runners
+    or build trees compares equal.
+
+    Rule: strip everything before (and including) the last ``/src/snmalloc/``
+    occurrence, after replacing backslashes with forward slashes. Paths
+    without ``/src/snmalloc/`` are kept verbatim (they are out-of-tree).
+    """
+    p = path.replace("\\", "/")
+    needle = "/src/snmalloc/"
+    idx = p.rfind(needle)
+    if idx >= 0:
+        # Result begins with ``src/snmalloc/...`` (no leading slash).
+        return p[idx + 1:]
+    return p
+
+
+def parse_platform(doc: dict) -> dict[str, dict]:
+    """Convert a single llvm-cov JSON doc into
+    ``{normalised_path: {"executable": set[int], "covered": set[int],
+                          "regions_executable": int, "regions_covered": int}}``.
+
+    A line ``ℓ`` enters ``executable`` iff at least one segment on ``ℓ``
+    has ``has_count == true`` AND ``is_gap_region == false``. It enters
+    ``covered`` iff additionally at least one such segment has ``count > 0``.
+    By construction ``covered ⊆ executable``.
+    """
+    files: dict[str, dict] = {}
+    for entry in doc.get("data", []):
+        for f in entry.get("files", []):
+            fn = f.get("filename")
+            if not isinstance(fn, str):
+                continue
+            key = normalise_path(fn)
+            executable = files.setdefault(key, {
+                "executable": set(),
+                "covered": set(),
+                "regions_executable": 0,
+                "regions_covered": 0,
+            })
+            for seg in f.get("segments", []):
+                if not isinstance(seg, list) or len(seg) < _SEG_MIN_LEN:
+                    continue
+                if not seg[_SEG_HAS_COUNT]:
+                    continue
+                if seg[_SEG_IS_GAP]:
+                    continue
+                line = seg[_SEG_LINE]
+                if not isinstance(line, int):
+                    continue
+                executable["executable"].add(line)
+                if isinstance(seg[_SEG_COUNT], (int, float)) and seg[_SEG_COUNT] > 0:
+                    executable["covered"].add(line)
+            # Region totals: read from per-file summary (advisory only).
+            summary = f.get("summary", {})
+            regions = summary.get("regions", {})
+            r_count = regions.get("count")
+            r_covered = regions.get("covered")
+            if isinstance(r_count, int):
+                executable["regions_executable"] += r_count
+            if isinstance(r_covered, int):
+                executable["regions_covered"] += r_covered
+    return files
+
+
+def merge(platforms: dict[str, dict[str, dict]]) -> dict:
+    """Merge per-platform parsed maps into the canonical merged structure."""
+    all_files: set[str] = set()
+    for pmap in platforms.values():
+        all_files.update(pmap.keys())
+
+    merged_files: dict[str, dict] = {}
+    for fn in sorted(all_files):
+        executable: set[int] = set()
+        covered: set[int] = set()
+        for pmap in platforms.values():
+            entry = pmap.get(fn)
+            if entry is None:
+                continue
+            executable |= entry["executable"]
+            covered |= entry["covered"]
+        # Defensive: assert invariant covered ⊆ executable
+        assert covered <= executable, f"invariant violation for {fn}"
+        merged_files[fn] = {
+            "executable": sorted(executable),
+            "covered": sorted(covered),
+        }
+
+    total_exec = sum(len(v["executable"]) for v in merged_files.values())
+    total_covered = sum(len(v["covered"]) for v in merged_files.values())
+
+    plat_out: dict[str, dict] = {}
+    for label, pmap in platforms.items():
+        files_view = {
+            fn: {
+                "executable": len(entry["executable"]),
+                "covered": len(entry["covered"]),
+                "regions": {
+                    "executable": entry["regions_executable"],
+                    "covered": entry["regions_covered"],
+                },
+            }
+            for fn, entry in pmap.items()
+        }
+        p_total_exec = sum(v["executable"] for v in files_view.values())
+        p_total_covered = sum(v["covered"] for v in files_view.values())
+        p_total_r_exec = sum(entry["regions_executable"] for entry in pmap.values())
+        p_total_r_covered = sum(entry["regions_covered"] for entry in pmap.values())
+        plat_out[label] = {
+            "files": files_view,
+            "totals": {
+                "executable": p_total_exec,
+                "covered": p_total_covered,
+                "regions": {
+                    "executable": p_total_r_exec,
+                    "covered": p_total_r_covered,
+                },
+            },
+        }
+
+    return {
+        "files": merged_files,
+        "totals": {"executable": total_exec, "covered": total_covered},
+        "platforms": plat_out,
+    }
+
+
+# --- Markdown rendering ------------------------------------------------------
+
+def md_escape(s: str) -> str:
+    """Escape a filename for inclusion in a markdown table cell."""
+    return s.replace("|", r"\|").replace("\r", " ").replace("\n", " ")
+
+
+def _pct(covered: int, executable: int) -> str:
+    if executable == 0:
+        return "n/a"
+    return f"{100.0 * covered / executable:.2f}%"
+
+
+def _top_dir(path: str) -> str:
+    """Group key for the per-directory table.
+
+    For paths under ``src/snmalloc/``, group by the immediate sub-directory
+    (e.g. ``src/snmalloc/pal/...`` → ``src/snmalloc/pal``). Other paths are
+    grouped under ``other``.
+    """
+    if path.startswith("src/snmalloc/"):
+        rest = path[len("src/snmalloc/"):]
+        head, _, _ = rest.partition("/")
+        return f"src/snmalloc/{head}" if head else "src/snmalloc"
+    return "other"
+
+
+def _in_scope(path: str) -> bool:
+    """Filter a normalised path to ``src/snmalloc/**`` (excluding tests and
+    concept headers). Same scoping as ``.copilot/coverage_diff.py``."""
+    if not path.startswith("src/snmalloc/"):
+        return False
+    if path.endswith("_concept.h"):
+        return False
+    return True
+
+
+def render_markdown(merged: dict) -> str:
+    out: list[str] = []
+    # Marker used by .github/workflows/coverage-comment.yml's
+    # find-or-create logic (see the dual-marker policy: comment must
+    # be authored by github-actions[bot] AND its body must contain
+    # this marker). If you change this string you MUST update both
+    # occurrences in coverage-comment.yml in lockstep, or comment
+    # dedup silently breaks (every run posts a new comment).
+    out.append("<!-- snmalloc-coverage-bot -->")
+    out.append("## Coverage report (cross-platform merged)")
+    out.append("")
+    # Headline is in-scope (``src/snmalloc/**``) only — that is the project
+    # code being measured. The JSON artifact retains the full unfiltered
+    # data for downstream consumers (e.g. ``coverage_diff.py``).
+    scoped_exec = 0
+    scoped_cov = 0
+    for fn, v in merged["files"].items():
+        if not _in_scope(fn):
+            continue
+        scoped_exec += len(v["executable"])
+        scoped_cov += len(v["covered"])
+    out.append(
+        f"**Lines covered (`src/snmalloc/**`): {scoped_cov} / {scoped_exec} "
+        f"({_pct(scoped_cov, scoped_exec)})**"
+    )
+    out.append("")
+    out.append(
+        "_Merged line coverage is the per-line union across all platforms. "
+        "Region coverage is reported per-platform only; no cross-platform "
+        "region total is computed._"
+    )
+    out.append("")
+
+    # Per-directory breakdown (in-scope only).
+    dir_totals: dict[str, dict[str, int]] = {}
+    for fn, v in merged["files"].items():
+        if not _in_scope(fn):
+            continue
+        d = _top_dir(fn)
+        bucket = dir_totals.setdefault(d, {"executable": 0, "covered": 0})
+        bucket["executable"] += len(v["executable"])
+        bucket["covered"] += len(v["covered"])
+
+    out.append("### Per-directory breakdown")
+    out.append("")
+    out.append("| Directory | Lines covered | Lines executable | % |")
+    out.append("| --- | ---: | ---: | ---: |")
+    rows = sorted(
+        dir_totals.items(),
+        key=lambda kv: (
+            (kv[1]["covered"] / kv[1]["executable"]) if kv[1]["executable"] else 1.0,
+            kv[0],
+        ),
+    )
+    for d, t in rows:
+        out.append(
+            f"| {md_escape(d)} | {t['covered']} | {t['executable']} | "
+            f"{_pct(t['covered'], t['executable'])} |"
+        )
+    out.append("")
+
+    # Per-platform contributions (advisory).
+    out.append("<details><summary>Per-platform contributions (advisory)</summary>")
+    out.append("")
+    out.append("| Platform | Lines covered | Lines executable | Lines % | Regions covered | Regions executable | Regions % |")
+    out.append("| --- | ---: | ---: | ---: | ---: | ---: | ---: |")
+    for label in sorted(merged["platforms"].keys()):
+        pt = merged["platforms"][label]["totals"]
+        rt = pt["regions"]
+        out.append(
+            f"| {md_escape(label)} | {pt['covered']} | {pt['executable']} | "
+            f"{_pct(pt['covered'], pt['executable'])} | "
+            f"{rt['covered']} | {rt['executable']} | "
+            f"{_pct(rt['covered'], rt['executable'])} |"
+        )
+    out.append("")
+    out.append("</details>")
+    out.append("")
+    return "\n".join(out)
+
+
+# --- CLI ---------------------------------------------------------------------
+
+def parse_inputs(spec_list: list[str]) -> dict[str, Path]:
+    inputs: dict[str, Path] = {}
+    for spec in spec_list:
+        if "=" not in spec:
+            raise SystemExit(f"error: input must be LABEL=PATH, got {spec!r}")
+        label, _, path = spec.partition("=")
+        if not label or not path:
+            raise SystemExit(f"error: empty label or path in {spec!r}")
+        if label in inputs:
+            raise SystemExit(f"error: duplicate label {label!r}")
+        inputs[label] = Path(path)
+    return inputs
+
+
+def load_doc(path: Path) -> dict:
+    try:
+        with path.open() as f:
+            doc = json.load(f)
+    except (OSError, json.JSONDecodeError) as exc:
+        raise SystemExit(f"error: cannot load {path}: {exc}")
+    if not isinstance(doc, dict) or "data" not in doc:
+        raise SystemExit(f"error: {path} missing top-level 'data' key")
+    return doc
+
+
+def main(argv: list[str] | None = None) -> int:
+    ap = argparse.ArgumentParser(description=__doc__)
+    ap.add_argument("--output-json", required=True, type=Path)
+    ap.add_argument("--output-md", required=True, type=Path)
+    ap.add_argument("inputs", nargs="+", help="LABEL=PATH pairs")
+    args = ap.parse_args(argv)
+
+    spec = parse_inputs(args.inputs)
+    platforms: dict[str, dict[str, dict]] = {}
+    for label, path in spec.items():
+        platforms[label] = parse_platform(load_doc(path))
+
+    merged = merge(platforms)
+
+    args.output_json.write_text(json.dumps(merged, indent=2, sort_keys=True))
+    args.output_md.write_text(render_markdown(merged))
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())